By default, your store is set up so that customers can create accounts at the very end of the checkout process.

Once a customer completes his first order, he can type in a password on the Thank You page to create an account. The shipping address form will be pre-filled on subsequent orders provided that the customer logs in to his account.

If this does not work for your shop, check to make sure that customer accounts are enabled. To do this, go to the Settings > Checkout section of your admin panel, and scroll down to "Customer Accounts." 

Make sure "Accounts are optional" is checked off. Click "save."


 

Because the sign up section is AFTER the checkout, your customers:

• don't have to lift a finger to fill out their sign-up info – it's already been provided during check out! 
• will be more tempted to want to "belong" to your brand as they've already invested money in it.

This method is also beneficial for you, as you won't have to deel with accounts from people who never buy from your store and aren't actually customers. 

I'm worried my customers will want to sign up immediately.

E-commerce research shows that the likeliness of someone creating a customer account without purchasing something first is very slim. Shopify minimizes how many loops a client needs to go through to purchase something and register.

The best thing to do to relieve any potential confusion is to inform customers about how they can get a Customer Account. Example: include an "Ordering Info" page and link it to your footer or Product Description or Customer Login page.

If you do want to include a "sign up" button on your main page, here's how:

You can add a customer registration form that allows your visitors to sign up for accounts. Check out this guide to allow your visitors to sign up for accounts.